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Anesthesia drugs for the small animal patient (Proceedings)

The development of a successful anesthesia plan is built on an understanding of several factors. The patient's physiological needs, the demands of the procedure that necessitates the anesthesia, the capabilities of the personnel involved in the procedure and the resources available to the anesthetist, all have a role. An important resource for the anesthetist is a good knowledge and understanding of the pharmacology of the anesthesia and adjunct drugs used to provide anesthesia care. Also important is an understanding of the principles and techniques of their administration.

There are a variety of drugs and techniques used for anesthesia management in modern veterinary medicine today. Unfortunately, no one anesthetic drug is appropriate for every patient, under all conditions. All anesthetic drugs have the potential for undesirable effects. The goal for the anesthetist is to understand which drugs positive effects outweigh the negative effects for a given condition of the patient. Which anesthetic drugs are most likely to provide the best support that will result in a successful anesthesia course and uneventful recovery?

Anticholinergics

The use of anticholinergics in an anesthetic plan is a safe and effective way to avoid the effects of parasympathetic reflexes. Although their use is controversial, they can be a very effective tool for the anesthetist. Anticholinergics are used to block vagal reflexes, preventing cardiac dysrythmias, decreasing secretions, and reducing the risks associated with gastric reflux during anesthesia. At higher doses they can cause tachycardia and increase myocardial oxygen consumption and decrease GI motility, or at low doses cause A-V Block. Most anesthetic drugs elicit a cholinergic response from the patient's autonomic nervous system; however the indiscriminate use of anticholinergics in every patient is not appropriate. These drugs are contraindicated in patients with pre-existing tachycardia as seen in thyrotoxicosis or fever.

Phenothiazines

Phenothiazines such as acepromazine are used to sedate patients for simple non painful procedures, relieve patient anxiety or as premedication for general anesthesia. They provide good sedation and reduce the amount of anesthetic agents needed to induce and maintain anesthesia. Acepromazine reduces the occurrence of catecholamine induced arrhythmias, causes vasodilation that can result in hypotension, and in some breeds such as the boxer has been associated with cardiovascular collapse. Acepromazine is not reversible and has long acting effects. These effects may be prolonged in patients with reduced liver function or when administered to patients at higher doses. Inhibition of platelet aggregation and mild respiratory depression has also been reported with the use of this drug.

Benzodiazepines

Benzodiazepines produce a calming or decreased anxiety effect, muscle relaxation, and provide excellent anticonvulsant effects. These drugs are frequently used as anticonvulsants for seizure control, as tranquilizers that provide muscle relaxation and sedation, and for behavior modification to controlling fear-induced behaviors. The effects of these drugs can be reversed with flumazenil. Benzodiazepines enhance the action of GABA (gamma-aminobutyric acid), the major inhibitory neurotransmitter in the brain. When benzodiazepines are used 1) as a preanesthetic agent or 2) with an opioid for neuroleptic sedation, or 3) with an anesthesia induction drug as an anesthesia co-induction agent; the quantity of other anesthetic drugs required to both induce and maintain adequate anesthesia is reduced. They have minimal cardiopulmonary effects; the analgesic effects from benzodiazepine drugs are minimal. The negative effects would include: excitement in cats and some dogs and prolonged effects in patients with liver disease. Diazepam is not water soluble which may result in unpredictable results due to unreliable absorption when administered IM or SQ. Administration of diazepam IM can cause pain and possibly the development of a sterile abscess at the injection site.

Analgesics

The assessment of pain represents one of the greatest challenges in veterinary medicine. Since our patients cannot verbally communicate their pain, we must rely on observations to evaluate the animal's comfort level. Various studies have been performed trying to establish a satisfactory objective scale to consistently assess pain in animals without success.

The management of pain has moved to the forefront in veterinary medicine. The focus is to prevent pain perception rather than to treat pain once the patient has become cognoscente of it. Pain can be much more difficult to control once the patient has become conscious of it. The use of analgesics is becoming more prevalent in the practice of veterinary medicine. Therapy plans have evolved over the past several years to include the use of local anesthetic drugs, opioids, alpha2-adrenergic agonists, NSAIDs, or combinations of these drugs to provide good analgesic levels and patient comfort.